This invention relates generally to a semiconductor device and specifically to a directional emission light emitting diode (“LED”) array with microoptic elements.
Semiconductor light emitting diodes are semiconductor chips that are mounted in a package and emit radiation in response to an applied voltage or current. These LEDs are used in a number of commercial applications such as automotive, display, safety/emergency and directed area lighting. A high brightness is desired for these applications.
White light emitting LED arrays are made by placing a plurality of LEDs into a package, where the individual LEDs emit different color lights. For example, by including red, green and blue emitting LEDs into an array, their combined output appears white to a human observer. Other LED arrays contain LEDs which emit only one color of light to form a solid state lighting device which emits a particular color of light.
However, the conventional LED arrays used for lighting applications suffer from several disadvantages. The LEDs used in conventional lighting applications emit light in all directions. Therefore, such LEDs must be mounted in a cup-shaped cavity in a carrier. The cavity contains a reflective material on its sidewalls to reflect the light toward the viewer. Furthermore, each conventional LED also requires a large dome lens to efficiently extract the light from the reflective carrier cavity. The cup-shaped carrier cavity and the dome lens increase the size of the package of each LED (i.e., increase the LED “form factor”). Therefore, the number of LEDs per square inch, and hence the light emitting density of the LED array, is decreased.
LED arrays that contain LEDs which emit different colors also require external optics that mix the individual colors to produce the desired single color output. For example, the external optics mix red, green and blue LED emission to obtain a white output. However, the external optics are relatively large. Therefore, the individual LEDs in the array have to be spaced apart at an undesirably large distance in order for the external optics to work properly. Therefore, the external optics require a smaller than desired number of LEDs per square inch, and hence the light emitting density of the LED array is decreased when the external optics are used.
FIG. 1 illustrates a conventional lateral current injection, top emitting (if the top electrode 17 is semi-transparent) or substrate emitting (if the top electrode 17 is reflective) GaN/InGaN LED 1. FIG. 2 illustrates a conventional LED array 21 with an undesirably high form factor due to the presence of the cup, dome lens and external optics. The conventional lateral current injection LED 1 contains a sapphire substrate 3, a GaN buffer layer 5, an n-type GaN contact layer 7, a GaInN (Ga0.55In0.45N, for example) quantum well active layer 9, a p-type AlGaN (Al0.2Ga0.8N, for example) barrier layer 11, a p-type GaN contact layer 13, a first electrode 15 which contacts the n-type contact layer 7 and a second electrode 17 which contacts the p-type contact layer 13.
This LED is a lateral current injection LED because the current is injected from the first electrode 15 into the active layer 9 laterally, since the first electrode 15 is laterally spaced from the active layer 9. Thus, in a lateral current injection LED, the active layer 9 and the first electrode 15 are located over the same (i.e., top) surface 19 of the lower contact layer 7. For example, in FIG. 1, the active layer 9 is located over the first lateral portion of the top surface 19 of the n-type contact layer 7 and the first electrode 15 contacts a second lateral contact portion of the top surface 19 of the n-type contact layer 7. In contrast, in a vertical current injection LED, the active layer and the first electrode are located over opposite sides of the lower contact layer and the current is injected vertically into the active layer, since the electrodes are located on opposite sides of the LED.
The packaged LED array 21 shown in FIG. 2 contains a plastic carrier 22 with leads 23 connected to the individual vertical current injection LED chips 24, 25 and 26. For example, LED 24 may be a red emitting LED, LED 25 may be a green emitting LED and LED 26 may be a blue emitting LED. The carrier 22 contains a plurality of cup-shaped LED carrier cavities 27. The sidewalls of the cavities are at least partially coated with a reflective metal 28. A relatively large dome lens 29 is placed over each LED and external optics 30 are located above the package 22. Thus, as illustrated in FIG. 2, the LEDs 24, 25 and 26 have to be located a relatively large distance apart in order to accommodate the reflector 28 coated cavities 27, the dome lenses 29 and the external optics 30. The present invention is directed to overcoming or at least reducing the problems set forth above.